non-LDS student the agreement is that they will follow the honor code and get annual ecclesiastical endorsements from their minister and in turn, BYU provides them with a place and a somewhat subsidized education. For members of the Church, who need the support of their seminary teacher(s) and stake president in order to even be considered for admission, the agreement is they will follow the honor code, which for them includes a provision about being active in the Church, and get an annual ecclesiastical endorsement from their Church leader. In return, the Church/BYU gives them a place at the university and an education that is heavily subsidized by the Church (tuition pays about 1/3 of the cost; the Church (anyone paying tithing) foots the bill for the other 2/3).
When a student who is a member of the Church leaves the Church, they have broken the contract (and the commitments they have made); this voids the contract. It is illogical to think that once one party of the contract has not fulfilled their part, that the other party should still continue to uphold its part. Furthermore, no one who no longer sees himself/herself as a member of the Church is forced to lie about their beliefs - they decide on their own to act unethically and continue to take what the Church offers while not fulfilling their commitment and their part of the bargain. An honest person admits that they can no longer fulfill their part of the contract and withdraws; lying in order to receive benefits to which you are no longer entitled is unethical.
Every time this topic comes up on CB, I am amazed by how many people think it is okay to ignore commitments and expect to reap the benefits anyway. Does a person's word and promise mean anything anymore? Is integrity no longer important?
As for the idea that someone who leaves the Church should be able to stay and pay the non-member tuition, two points should be considered. First, once a person has broken the original contract, why should the Church be compelled to enter into a second one? What is in it for the Church? This leads to the second point: the reason the Church is in the education business - and I have heard this directly from 2 members of the BOT - is to build up the leadership of the Church. In other words, the purpose of BYU is to strengthen the youth and prepare them not only for careers in the secular world, but also for a lifetime of activity, service, and leadership in the Church. When a student affirms that they no longer want to be a part of building the Kingdom, the Church will, as is its right, give that place at the university to someone who will.